The method of irradiation by pulses shorter than the phase relaxation time of the matter to be excited (collision time of particles in a gas) yields better selectivity for the same amount of excitation of a selected particle than conventional irradiation by continuous light or longer pulses. Such relaxation times or collision times are usually on the order of picoseconds to nanoseconds with a practical upper limit of 10 nanoseconds. Accordingly this invention relates to an improved method and apparatus for the efficient excitation, acceleration and trapping of particles by means of light radiation pressure, which particles are otherwise free to move with respect to their environment, exemplified by separation of isotopes. Previous methods utilizing light irradiation of an isotopic mixture for separation of a preselected isotope are many and varied. Examples include Ashkin U.S. Pat. No. 3,710,279, Pressman U.S. Pat. No. 3,558,877 and Braunstein et al, U.S. Pat. No. 3,532,879 in which isotopic separation is obtained through irradiation by a continuous wave of coherent light at the absorption wavelength of one of the isotopes. These methods are inefficient in that:
(a) The ratio of the kinetic energy acquired by the atom or molecule of mass M compared to the energy .nu. of the absorbed proton is small. The ratio is given by .eta.= .nu./2Mc.sup.2 and is less that 10.sup.-7. PA1 (b) In order to excite a large fraction of the selected isotopes per unit time, high intensities are required for the exciting radiation. However, even at the highest intensities, corresponding to saturation, no more than 50% of the isotopes to be separated can be excited at one time. PA1 (c) The greater the intensity, the broader the energy spectrum excited by the irradiation, an effect known as "power broadening". Thus, as higher intensity light is used to maximize the excitation rate of the isotopes to be excited, the less selective the radiation is with respect to the other isotopes. Accordingly, the energy required for a high isotope production rate conflicts with a requirement for high selectivity. PA1 (d) On photon (at most) is used--only once--to push an atom or molecule. The quantum of efficiency is thus limited to .nu./2Mc.sup.2.
The present invention overcomes the foregoing disadvantages by using a train of ultrashort pulses as the irradiating light and by disposing particles (atoms or molecules) to be accelerated, or a mixture of particles to be separated, in the end cavity of a mode-locked laser. The laser pulses each have a duration shorter than the phase relaxation time of the particles to be accelerated. Particles which are resonant with the frequency of the laser beam pulses are excited, giving them a momentum in the direction of the beam. As the pulses are reflected back through the cavity, the previously excited particles restitute their energy to the reflective pulses by stimulated emission. The restituting particle emits in the direction of travel of the reflected pulse, receiving additional momentum in the opposite direction so that it accelerates further in the direction of the original pulse. The accelerated particles thus can be spatially isolated and can be collected using known techniques.
Also disclosed are three pulse configurations, each of which interreacts with the particles contained in the particle mixture differently, but all of which result in a net momentum imparted to the particle to be accelerated. One pulse configuration is essentially "bell shaped" (Gaussian). When this pulse is used for "single photon" absorbing transitions, the pulse is tuned at resonance with a transition of the desired particle to be excited and accelerated. When it is used for "two-photon transitions", it is tuned close to resonance, with half of the transition frequency of the desired particle to be excited and accelerated. In addition, the "bell shaped" pulse can be configured to contain two frequencies to use with two-photon transitions. A second pulse configuration termed "zero-area" pulse applies only to direct "single-photon" absorbing transitions, and is tuned at resonance with an unwanted particle (the particle which is not to be excited--and--in the particular case of the proposed radiation pressure apparatus--which is not to be accelerated). The "zero-area" pulse has a frequency spectrum sufficiently broad to contain energy at the light absorption wavelength of the desired particle (to be excited and--in the particular case of the radiation apparatus described here--accelerated). The third pulse configuration is the "two-photon" transition correspondent of the "zero-area" pulse and is termed "90.degree. phase shifted", because half of that pulse is 90.degree. out of phase with the other half. The "90.degree. phase shifted" pulse is tuned close to resonance with half of the transition frequency of the unwanted element (not to be excited or accelerated).